Cerebral palsy (CP) is known to be associated with disorders of the visual system including refractive errors, poor visual acuity, accommodative dysfunction, oculo-motor disorders, and cerebral visual impairment. The incidence of these disorders varies greatly among studies, depending in particular on the selection criteria (population-based studies, clinical trials, etc.) and on the source of clinical information (registers, hospital records, direct testing, etc.). Yet the presence of a visual disorder of any kind in children with CP is considered quite common. This is not surprising when looking on the one hand, at the populations at risk of CP, such as the very preterm born infants or asphyxiated term infants, who share similar risk factors for brain damage and ocular disorders, and on the other hand, at the wide extent of the distributed network of vision-related brain areas at high risk of being affected in case of congenital brain damage.
Despite their high incidence, visual disorders are still difficult to identify in children with CP. Many means of visual assessment require reliable behavioral responses, which are not easy to elicit and interpret by observers not specifically trained. Also, the functional consequences of visual disorders are often transformed or disguised by concomitant motor or cognitive impairments, making it hard to appreciate their actual contribution to the overall clinical picture. In other words, many visual disorders share the discouraging properties of being hard to measure and difficult to decipher in their functional consequences, thus making them often under-recognized. A clear example of such difficulties is presented in the paper by Pansell et al., who report on the disorders of the accommodative process in children with CP, providing important new insights in the field.
It is of interest that the focus of the Pansell et al. paper was on children with CP with normal or near normal best-corrected visual acuity and visual fields, in combination with an ability to cooperate in the test. In spite of the selection of a relatively lower risk subgroup, children with CP were found to accommodate significantly less than the typically developing age-matched controls, suggesting an even larger problem in the general population with CP. By measuring objectively and independently accommodation and pupillary response, the authors were able to shed some new light on the different strategies that children with CP put in place to obtain clear vision at near, by mixing different combinations of accommodation and pupil constriction. In contrast with previous beliefs, these new findings suggest that near pupil response alone cannot be used as an effective indicator of accommodative function, but that the two functions act independently and thus an objective assessment of accommodation is always needed.
A further novel finding emerging from the work by Pansell et al. is that the participants with dyskinetic CP were the most impaired in terms of visual acuity at near, as a result of the association of poor accommodative response and reduced pupil constriction. This is in keeping with a number of other reports suggesting a key role for the basal ganglia and thalami in the development of visual functions in newborns, with brain damage both in high-risk preterm infants and in term infants with birth asphyxia. Evidence is growing in support of the concept that sparing of these subcortical nuclei is an essential requirement for early brain plasticity of both perceptual and motor functions to be effective.
In summary, it is certainly safe to affirm that enough evidence has been gathered supporting a pivotal role of visual functions in both neuromotor and cognitive development in children with CP. Reduced accommodative function, for example, can result in reduced visual acuity or persistent blur in near tasks, such as reading or writing, with obvious effects on learning processes. As a clear consequence, an early and accurate assessment of visual disorders is critical in the context of neurorehabilitation of CP. It undoubtedly contributes to psychomotor improvement and to an overall better outcome. The role of scientists and health care providers is to work relentlessly in order to make this complex picture progressively less ‘blurred’.